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Right, here's some context. I have a large amount of metal shavings from a dozen different sources, and they are very light and sharp. Some of them aren't, but most of them are. I need to dispose of them without ruining the day of an unlucky employee at a recycling plant. So I intend to melt them into easily organized and transportable ingots.

The only problem is, I also want to learn how to make an induction heater, and all of those metal shavings sound like perfect test subject for this project.

Now, when it comes to making an induction heater, my research has informed me that I need a Tank Capacitor hooked up to an oscillating power supply. The Tank Capacitor part sounds simple enough, I have serviceable capacitors for days, and I have copper pipe from a plumbing project that went nowhere, for the work coil.

The project got complicated when the concept of resonance got introduced. A quick wiki-crawl and an examination of several similar projects revealed that a Tank Capacitor has a specific resonance frequency, where the energy gain (Transfer? Whatever.) is at it's most efficient. The equation can be found with a quick google. However, the inductance, one of the variables used to determine the resonance frequency, changes based on the contents of the work coil. I have been led to believe that it also changes based on how hot the work piece is.

So I have to ask, how do I determine the resonant frequency while the work coil is functioning? Can I just attach a wire to the Tank Capacitor to determine the frequency that its oscillating at, and get the resonant frequency from that?

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  • \$\begingroup\$ employees at a recycling plant already expect everything to be sharp. i do not think that you will surprise them. \$\endgroup\$ – jsotola Mar 16 '18 at 22:46
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    \$\begingroup\$ It's less about cutting someone, and more about being impossible to organize if they cut their way out of the bags. \$\endgroup\$ – I'mNotSure Mar 16 '18 at 22:58
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    \$\begingroup\$ The inductance & capacitance together form a resonant 'tank circuit'. A 'Tank Capacitor' isn't really a thing, and doesn't really have a resonant frequency of its own (at least not one relevant to what you're trying to do). \$\endgroup\$ – brhans Mar 17 '18 at 2:28
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    \$\begingroup\$ You can just assemble your work coil and capacitor, and stimulate them with a step function, then see what frequency they resonate at. The easiest way to do the step function would be to connect a small battery (like a 9V transistor battery), then remove it and watch the sine wave with an oscilloscope. You can do this with and without metal in the coil. You are not going to be actually heating up the work piece. Just observing the frequency. It doesn't matter that it is a low voltage. \$\endgroup\$ – mkeith Mar 17 '18 at 4:27
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    \$\begingroup\$ Having large unknown alloy chunks will ruin his day more than shavings that he can sort if he wants \$\endgroup\$ – PlasmaHH Mar 17 '18 at 8:58
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You seem to be asking how to measure the frequency once you've already gotten it oscillating. That's much easier than getting the circuit oscillating properly in the first place, but so be it. I would recommend a small pickup coil a safe distance from your heating coil. Send it to your counter or oscilloscope and you should see a nice signal. Now for dimensions, I'd take a guess that at 1 MHz and with a 5 kW heating power you'd probably want a coil area of about 1 cm squared and about 10 turns. Insulate it so you can't burn out your counter/scope if something goes wrong.

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  • \$\begingroup\$ Wait, so the Tank Circuit always oscillates at its resonant frequency no matter the input? \$\endgroup\$ – I'mNotSure Mar 17 '18 at 17:23
  • \$\begingroup\$ The tank circuit has a natural resonant frequency. The best way to describe it is to say that it always WANTS to resonate at that frequency when it is stimulated in any way. Similar to a string on a guitar. However you strum it, the note it plays is basically the same. If you drive a resonant circuit near its resonant frequency, the impedance will be at its lowest. As you move away from the resonant frequency, the impedance rises. In principal, if you have enough voltage available, you can drive a coil without a capacitor. \$\endgroup\$ – mkeith Mar 17 '18 at 19:01
  • \$\begingroup\$ 1 MHz 5 kW. That would be a serious piece of equipment. \$\endgroup\$ – mkeith Mar 18 '18 at 20:18
  • \$\begingroup\$ The skin depth is only 82 um in aluminum at 1 MHz. \$\endgroup\$ – mkeith Mar 18 '18 at 20:20
  • \$\begingroup\$ electrogas' advice helped, but I'm not going to go that high in terms of power. I only have a 1.8 kW power source. \$\endgroup\$ – I'mNotSure Mar 19 '18 at 0:30

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